I. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention relates to a novel and improved method for hard handoff between different wireless communication systems.
II. Description of the Related Art
In a code division multiple access (CDMA) spread spectrum communication system, a common frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Interim Standard IS-95-A entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,901,307 entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITE OR TERRESTRIAL REPEATERS" and in U.S. Pat. No. 5,103,459 entitled "SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM" both of which are assigned to the assignees of the present invention and incorporated herein by reference.
Signals occupying the common frequency band are discriminated at the receiving station by using a high rate pseudonoise (PN) code. The PN code modulates signals transmitted from the base stations and the mobile stations. Signals from different base stations can be separately received at the receiving station by discrimination of the unique time offset that is introduced in the PN codes assigned to each base station. The high rate PN modulation also allows the receiving station to receive a signal from a single transmission station where the signal has traveled from the the base station to the receiving station over several different distinct propagation paths (commonly referred to as "multipathing"). Demodulation of multipath signals is disclosed in U.S. Pat. No. 5,490,165 entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS" and in U.S. Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The use by all base stations within a particular system of a common frequency band allows simultaneous communication between a mobile station and more than one base station, a condition known as soft handoff. On implementation of a soft handoff method and apparatus is disclosed in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM" and U.S. Pat. No. 5,267,261 entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM", both assigned to the assignee of the present invention and incorporated herein by reference. Similarly, a mobile station can be simultaneously communicating with two sectors of the same base station, known as softer handoff as disclosed in copending U.S. Patent Application entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION", U.S. Pat. No. 5,625,876, issued Apr. 29, 1997, assigned to the assignee of the present invention and incorporated herein by reference. An important feature is that both soft handoffs and softer handoffs make the new connection before breaking the existing one.
If a mobile station travels outside the boundary of the system with which it is currently communicating, it is desirable to maintain the communication link by transferring the call to a neighboring system, if one exists. The neighboring system may use any wireless technology, examples of which are CDMA, NAMPS, advanced mobile phone service (AMPS), time devision multiple access (TDMA), or global mobile systems (GSM). If the neighboring system uses CDMA on the same frequency band as the current system, an inter-system soft handoff can be performed. In situations where inter-system soft handoff is not available, the communication link is transferred through a hard handoff where the current connection is broken before a new one is made. Examples of typical hard handoff situations include: (1) the situation in which a mobile station is traveling from region serviced by a CDMA system to a region serviced by a system employing an alternate technology and (2) the situation in which a call is transferred between two CDMA systems which use different frequency bands (interfrequency hard handoff).
Inter-frequency hard handoffs can also occur between base stations of the same CDMA system. For example, a region of high demand such as a dense urban area may require a greater number of frequencies to service demand than the suburban region surrounding it. It may not be cost effective to deploy all available frequencies throughout the system. A call originating on a frequency deployed only in the high congestion area must be handed off as the user travels to a less congested area. Another example is system which encounters interference from another service operating on an interferring frequency within the system's boundaries. As users travel into an area suffering from interference from another service, their call may need to be handed off to a different frequency.
Handoffs can be initiated using a variety of techniques. Handoff techniques, including those using signal quality measurements to initiate a handoff, are found in copending U.S. Pat. No. 5,697,055 entitled "METHOD AND APPARATUS FOR HANDOFF BETWEEN COMMUNICATION SYSTEMS", U.S. Pat. No. 5,999,816, issued Dec. 9, 1997, assigned to the assignee of the present invention and incorporated herein by reference. Further disclosure of handoffs, including measurement of round-trip signal delay to initiate handoff, is disclosed in copending U.S. Pat. No. 5,848,063 entitled "METHOD AND APPARATUS FOR HARD HANDOFF IN A CDMA SYSTEM", issued Dec. 8, 1998, assigned to the assignee of the present invention and incorporated herein by reference. Handoffs from CDMA systems to alternate technology systems are disclosed in copending U.S. Pat. No. 5,594,718 entitled "METHOD AND APPARATUS FOR PROVIDING MOBILE UNIT ASSISTED HARD HANDOFF FROM A CDMA COMMUNICATION SYSTEM TO AN ALTERNATIVE ACCESS COMMUNICATION SYSTEM", issued Jan. 14, 1997, assigned to the assignee of the present invention and incorporated herein by reference. In the '718 patent, pilot beacons are placed at the boundaries of the system. When a mobile station reports these pilot beacons to the base station, the base station knows that the mobile station is approaching the boundary, and in response, prepares for the possibility of an intersystem hard handoff.
When a system has determined that a call should be transferred to another system via hard handoff, a message is sent to the mobile station directing it to do so along with parameters that enable the mobile station to connect with the destination system. The system from which the mobile station is departing has only estimates of the mobile station's actual location and environment, so the parameters sent to the mobile station are not guaranteed to be accurate. For example, with beacon aided handoff, the measurement of the pilot beacon's signal strength can be a valid criteria for triggering the handoff. However, the appropriate base station or base stations in the destination system which can effectively communicate with the mobile station are not necessarily known. These base stations with which the mobile can effectively communicate and which are considered to be good candidates based upon additional criteria are maintained in a list or "set" within the mobile station and commonly referred to as the "Active Set". Inclusion in the active set implies allocation of foward link resources. Allocation of all possible candidates as opposed to the minimal sufficient set is wasteful of system resources, reducing available system capacity. Even if all of the base stations in the destination system which can effectively communicate with the mobile station are known, including all such base stations may exceed the maximum number allowable in the Active Set.
In order for the mobile station to communicate with the destination system, contact with the old system must cease. If the parameters given to the mobile station are not valid for any reason (i.e. changes in the mobile station's environment or lack of precise location information at the base station) the new communication link will not be formed, and the call may be dropped. After an unsuccessful handoff attempt, the mobile station can revert back to the previous system if it is still possible to do so. However, with no further information and no significant change in the mobile station's environment, repeated attempts to perform a handoff may also fail.
A method for performing additional hard handoff attempts with greater probability of success is disclosed in copending U.S. Patent entitled "METHOD AND APPARATUS FOR PERFORMING MOBILE ASSISTED HARD HANDOFF BETWEEN COMMUNICATION SYSTEMS", U.S. Pat. No. 5,999,816, issued Dec. 7, 1999 assigned to the assignee of the present invention and incorporated herein by reference. In the 5,999,816 patent, mobile stations tune temporarily to the frequency of the hard handoff destination system and search for available pilot signals (hereafter referred to simply as "pilots") on that frequency, for inclusion of the associated base stations in the active set. After the searching task is completed, the mobile station will retune to the original frequency to resume current communications. While tuned to an alternate frequency, any frames of data generated by the mobile station or transmitted by the base station will be corrupted. Typically, the base station will provide only a subset of the possible offsets (commonly referred to as an "enable list") for the mobile station to search. Even so, the duration of the searching can be so long as to potentially corrupt a number of frames of data. As such, there is a need for searching techniques which minimize the duration of time that a mobile station spends tuned to an alternate frequency so as to minimize corruption of the active communication link. Such searching techniques will clearly improve the quality of systems employing the hard handoff techniques described above.